Wideband phased arrays are desirable for use in high-throughput communication systems, such as cellular and satellite systems, as well as radar systems, electromagnetic countermeasure systems, and multifunctional communications/sensing systems.
A popular wideband array element is the Vivaldi antenna, also called a “tapered slot antenna”, first proposed by Gibson in 1979 (P. J. Gibson, “The Vivaldi Aerial,” Proc. 9th European Microwave Conference, 1979, pp. 101-105.). The element consists of a flared slot structure that has been studied extensively since its inception, leading to theoretical and empirical developments that have extended its performance to achieve over a decade of bandwidth with good scan performance. For wideband scanning arrays the tapered slot has been the dominant technology. However, the Vivaldi elements have two main drawbacks: elements are large, typically a few wavelengths in size at the high end of the operating band, and are not modular since they require electrical connection between neighboring elements for good performance. It has been found that absence of electrical continuity between neighboring elements or subarrays introduces resonances that drastically reduce the achievable bandwidth (Schaubert, D. H.; Kasturi, S.; Boryssenko, A. O.; Elsallal, W. M., “Vivaldi Antenna Arrays for Wide Bandwidth and Electronic Scanning,” The Second European Conference on Antennas and Propagation, 2007(EuCAP 2007), pp. 1-6, 11-16 Nov. 2007).
A variation on the Vivaldi antenna is the Antipodal Vivaldi Antenna (AVA), introduced by Gazit in 1988 (E. Gazit “Improved design of the Vivaldi antenna,”Proc. IEEE Microw., Antennas Propag ., vol. 135, pp. 89, 1988.). The AVA consists of a microstrip line transitioning to a slotline structure with an exponential taper. This element has high cross pol due to the offset fins, but this was corrected with the addition of a third fin and the inception of the Balanced Antipodal Vivaldi Antenna (BAVA), introduced by Langely, Hall and Newman (J. D. Langely et al, “Balanced Antipodal Vivaldi Antenna for Wide Bandwidth Phased Arrays,” IEEE Proceeding of Microwave and Antenna Propagations, Vol. 143, No. 2 Apr. 1996, pp. 97-102.). Recently, Elsallal and Schaubert performed extensive numerical studies to understand and improve the performance of the BAVA element in dual and single polarized arrays (M. W. Elsallal and D. H. Schaubert, “Parameter Study of Single Isolated Element and Infinite Arrays of Balanced Antipodal Vivaldi Antennas,”2004Antenna Applications Symposium, Allerton Park, Monticello, Ill., pp. 45-69, 15-17 Sep., 2004.) and (M. W. Elsallal and D. H. Schaubert, “Reduced-Height Array of Balanced Antipodal Vivaldi Antennas (BAVA) with Greater than Octave Bandwidth,” Antenna Applications Symposium, Allerton Park, Monticello, Ill., pp. 226-242, 21-23 Sep., 2005.). The studies showed impedance anomalies occurring throughout the desired band that vastly limit the bandwidth of the array. Their studies led to solutions that allowed the anomalies to be controlled sufficiently for improved bandwidth, such as mirroring of elements in the E and H plane (called double mirroring) and placing slots in the fin layers, as described in the US Patent Application Publication 2008/02111726 (the DmBAVA-MAS), and a PhD thesis (M. W. Elsallal, “Doubly-Mirrored Balanced Antipodal Vivaldi Antenna (DmBAVA) for High Performance Arrays of Electrically Short, Modular Elements,” PhD dissertation, Electrical and Computer Engineering, Univ. of Massachusetts, February 2008). Among these advancements, the double mirroring is the key development that enables wideband operation. The BAVA achieves a wide bandwidth, low profile, and modularity, but requires a balun (180° hybrid) in the feed network. FIG. 1 shows the required phase shifter 23 with the DmBAVA that comprises a number of elements, each element including fin 1 and fin 2. Two adjacent elements are fed at opposite polarities using phase shifter 23.
A development similar to the BAVA is the bunny ear element, developed by J. J Lee (Lee, J. J., et al., “Wide Band Bunny-Ear Radiating Element,” Antennas and Propagation Society International Symposium, AP-S Digest, pp. 1604-1607, 1993, and U.S. Pat. No. 5,428,364). The element consists of a dielectric slab with a tapered slotline printed on each side that transitions from a narrow slot at the ground plane to a wide slot at the radiating aperture. The ground plane of the slotline is shaped into fins, with a narrow fin at the ground plane and a wide fin at the aperture. The element achieves wide bandwidth, is low profile, and is modular, but requires a balun embedded in the element, which increases the cost and complexity of fabrication.
The Bunny Ear Element is a balanced structure very similar to the dipole element, which has found extensive application in narrowband antenna arrays. The dipoles are low profile and modular, but the dipole is a balanced structure that also requires a balun in order to connect to standard RF interfaces. Thus the balun is a major design challenge, and much work has been done on the balun implementation. U.S. Pat. No. 3,747,114 issued to Shyhalla shows a dipole array with baluns printed on the backplane, with the balun consisting of phase delay lines between the balanced feed pins of the dipole elements. U.S. Pat. No. 6,512,487 issued to Taylor et al shows a dipole layer fed by balanced vertical feed pins, requiring an external balun. US Patent Application Publication US 2007/0222696 (Wikstrom et al.) and US Patent Application Publication US 2009/0051619 (Hook et al.) also involve arrays of dipoles fed directly by balanced transmission lines, requiring external baluns.
The necessity of the balun has led to much interest in developing integrated balun structures. One example is U.S. Pat. No. 3,845,490 issued to Manwarren et al, which shows a stripline dipole structure fed by an “L” shaped transmission line embedded between the dipole layers, forming a balun structure. U.S. Pat. No. 4,825,220 issued to Edward et al showed the use of a “J” shaped microstrip line feeding a microstrip dipole structure. U.S. Pat. No. 5,892,486 issued to Cook et al. also incorporated a “J” shaped microstrip line feeding a microstrip dipole where the “J” shaped balun extended above the dipoles. Each of these designs uses a balun consisting of an open circuited feed line proximity coupled to the dipole structure.